A cell that is not actively dividing is characterized as being in which phase?

A cell that is not actively dividing is characterized as being in interphase. Interphase is the stage of the cell cycle where the cell prepares for division but is not itself dividing. It consists of three sub-phases: G1 (gap 1), S (synthesis), and G2 (gap 2). During interphase, the cell is actively engaged in growth, metabolic activities, and the duplication of its DNA, which is crucial for the later phases of mitosis.

In contrast, the other phases mentioned—prophase, anaphase, and telophase—are all part of mitosis, the process of cell division itself. Prophase is the initial phase of mitosis where chromatin condenses into chromosomes and the nuclear envelope begins to break down. Anaphase follows, during which the sister chromatids separate and are pulled to opposite poles of the cell. Telophase is the final stage of mitosis where the chromosomes reach the poles, the nuclear envelope re-forms, and the cell prepares to complete division.

Thus, interphase is the correct answer because it denotes a period where the cell is not actively dividing, focusing instead on growth and preparation for the next division cycle.